Argon products are produced by separating the argon from air through the use of cryogenic rectification that is conducted within an air separation plant. The argon produced can be a crude argon product that is generally further processed to remove oxygen and nitrogen or a purified argon product containing very little oxygen.
In an air separation plant that is designed to produce argon, the air is first compressed and then purified of higher boiling contaminants such as water vapor, carbon dioxide, carbon monoxide and hydrocarbons. The resulting compressed and purified air stream is then cooled to a temperature suitable for its rectification within a distillation column system through indirect heat exchange with waste and product streams produced as a result of the rectification of air. This heat exchange is conducted in a heat exchanger, sometimes termed as the main heat exchanger, which can be a collection of heat exchangers having parallel flows of the air being cooled, subdivided between warm and cold ends and on the basis of the pressure of the product streams.
The compressed and purified air after having been cooled to a temperature at or near its dewpoint is then introduced into a higher pressure column thermally linked to a lower pressure column that operates at a higher pressure then the lower pressure column. A crude liquid oxygen column bottoms, sometimes referred to as kettle liquid and a nitrogen-rich vapor column overhead is produced in the higher pressure column. A stream of the crude liquid oxygen is then further refined in the lower pressure column to produce an oxygen-rich liquid column bottoms and a nitrogen-rich vapor column overhead. The oxygen-rich liquid column bottoms is partially vaporized against condensing the nitrogen-rich vapor produced in the higher pressure column to generate reflux for both of the columns. The distillation is conducted in either of the columns through mass transfer contact between descending liquid and ascending vapor phases within trays or packing contained within the columns. As the liquid phase descends within the lower pressure column, up to a point, it becomes richer in argon that has a similar volatility to the oxygen. At a point near which the argon concentration is a maximum, a stream of crude gaseous argon is removed and then introduced into an argon column to separate the argon from the oxygen and produce the argon product. Typically, the argon product is taken as a liquid from part of the reflux to the argon column. As can be appreciated, since argon is a value added product, it is desirable to control the air separation plant so that argon production will be at a maximum.
In U.S. Pat. No. 4,784,677 argon production is controlled by measuring the nitrogen concentration in the crude argon feed stream to the argon column and the oxygen content in the waste nitrogen stream. The flow rate of liquid nitrogen reflux fed to the lower pressure column is regulated on the basis of such measurements to control the nitrogen content in the crude argon feed stream. Decreasing the reflux rate will decrease the nitrogen content and vice-versa. A major purpose of such control is to prevent the nitrogen content in the argon column from being too large and thereby preventing a sufficient temperature difference in the argon condenser to condense reflux to the argon column and form the argon product. At an extreme, the argon column would not operate and will dump its liquid into its sump or back into the low-pressure column. A disadvantage of such a control scheme is that a change in reflux to the lower pressure column will not instantaneously change the nitrogen content in the crude argon feed stream. Moreover, when the reflux rate to the lower pressure column is reduced, the flow rate of the crude argon feed stream will also be reduced with a consequent reduction in argon production.
In U.S. Pat. No. 5,313,800, the nitrogen concentration in the crude argon feed to the argon column is not measured. Rather such concentration is derived by obtaining temperature measurements within the lower pressure column between the crude oxygen feed point and the location at which the crude argon feed is drawn. The derivation is obtained from a mathematical model correlating the temperature measurements with the nitrogen concentration within the crude argon feed stream. From such estimated content, the flow rate to the argon column can be controlled. Specifically, the crude liquid oxygen from the higher pressure column is fed to the argon condenser and is partially vaporized. Vapor and liquid phase streams produced as a result of such vaporization are fed to the lower pressure column. The flow of the vapor phase stream is controlled to in turn control the pressure within the argon condenser and therefore, the feed rate to the argon column in response to the computations of the nitrogen content of the crude argon column feed stream.
U.S. Pat. No. 7,204,101 uses a multivariable controller to maximize argon production. The controller operates to optimize argon recovery by maximizing the argon concentration in the crude argon column feed by decreasing the oxygen concentration in the feed while preventing concentration of the nitrogen from exceeding a controllable maximum. The controller functions by direct measurements of oxygen concentrations in such streams as the gaseous oxygen product, the crude argon column feed, the nitrogen stream produced by the lower pressure column, nitrogen reflux to the lower pressure column and nitrogen concentration in the crude argon feed stream and by controlling flow rates of the amount of air fed into the distillation column system, the gaseous oxygen product flow from the lower pressure column, the liquid nitrogen reflux to the lower pressure column and the flow of the crude argon fed to the argon column.
The problem with this type of control is that once nitrogen is seen at critical concentrations in the argon product, it is often too late to take effective control action to prevent the argon column from shutting down with a loss of argon production. As will be discussed, the present invention incorporates a control methodology that does not depend on any such direct measurements and therefore, allows an improved control of argon production that does not have to be as conservative as prior art control systems and therefore increased production of the argon.